Radiation detectors such as microbolometers operating at ambient temperatures operate by measuring a change in a sensor's resistance (typically a focal plane array FPA of sensors) caused by the heating effect of incident radiation. The desired radiation to measure at the FPA is referred to as scene radiation, whereas any other energy transfer to the FPA is referred to as non-scene radiation. Depending upon the type of sensor, certain non-scene radiation may register at the FPA as noise.
Both cryogenically cooled and uncooled (i.e.: not cryogenically cooled) detectors typically dispose a radiation sensor within a vacuum chamber or detector package that maintains pressures well below ambient levels. The vacuum chamber itself is generally defined by a window through which desired radiation passes, a base, and sidewalls. However, the two types of detectors employ different types of sensors.
Cryogenically cooled detectors employ photon sensors that measure electrical activity directly from incident radiation. An incident photon striking a photon sensor collides with a target that directly amplifies the incident photon by an avalanche or cascade process that generates a multiplicity of charge carriers that are detected as electrical entities in themselves. The same principal applies to ionized gas in a tube or solid state detectors that are cryogenically cooled. A photon sensor operates on a photoconductive effect and generates a photoconductive current.
Conversely, uncooled detectors such as bolometers employ thermal sensors whose resistance changes as a function of temperature. An external current applied across the thermal sensor is used to measure a change in electrical resistance through the sensor that results from absorbing heat from the incident radiation. A thermal sensor operates on a photoresistive effect and generates a photoresistive current. Examples of uncooled infrared detectors include Raytheon® thermal camera models 300D and 2000B and component detectors 2500AS and 2000AS. Thermal detectors are susceptible to package effects, which include heat from package walls (the confines of the vacuum chamber) interfering with sensing of incident radiation.
Typically, a FPA of thermal sensors is mounted to or fabricated on a readout integrated circuit (ROIC). Heat that accumulates in the FPA from scene and non-scene radiation is removed by a series of thermoelectric (TE) cooling elements or other heat extraction means, which may be within or outside of the vacuum chamber. A ceramic stage is disposed between the TE elements and the ROIC in order to provide structure for the TE cooling elements, and to ensure a sufficient thermal mass to minimize localized temperature spikes from non-scene radiation and to distribute non-localized temperature changes over time, affording the sensor a more thermally stable background. Non-scene radiation such as ambient heat emitted from the package or radiation of non-scene wavelengths may be sensed at the FPA. Due to the trend for smaller detectors, the close proximity of package walls and window to the FPA can result in non-scene heat causing a substantial increase in heat transfer from the package. This increase in heat can also be non-uniform, creating spatial noise within the FPA image as heat falls on different pixels of the FPA at different levels.
Prior attempts to overcome the problem of non-scene radiation being detected at the sensor have included pre-programming a variety of calibration settings for the ROIC that transfers signals from the FPA, each tailored to a specific package temperature range. As many as nine calibration settings have been employed, wherein the ROIC switches from one calibration setting to another as the package temperature and resulting heat flux on the FPA changes. As can be appreciated, this conventional approach adds cost and complexity.